This invention relates to magnetic brush development apparatus for use in developing the latent image in the dry electrophotographic process known as xerography. This process involves the formation of a latent electrostatic image on or in a photoconductor, such that a visible image can be produced by bringing dry, colored developing powder, called toner, having a proper polarity electrical charge, into physical contact with the photoconductor. Toner can be formulated to carry either a positive or a negative electrostatic charge.
Toner is often used as one constituent of a two-component developer mix. The other component is a relatively large particle called the carrier bead. These beads are selected from the triboelectric series such that agitation of the beads and toner causes a charge of the opposite polarity to reside on each. The small toner particles coat the carrier beads by electrostatic attraction.
One of the well known types of developing apparatus is the magnetic brush developing apparatus. In this apparatus the above-mentioned bead component of the developer mix is a magnetically permeable bead which is magnetically held against the surface of a rotating nonmagnetic tube, known as the brush roll. This mix is made to form a brush or bristle-like shape at the developing nip interface between the brush roll and a closely-spaced photoconductor. When this developer mix bristle-like shape is brought into physical contact with the photoconductor, the photoconductor's latent image, which is of opposite polarity to that of the toner, succeeds in causing toner to transfer from the carrier beads to the photoconductor, thus forming a visible toner image. The carrier beads are thus depleted of toner, and must be subsequently enriched with new toner which is added to the developer mix from a toner supply chamber or cartridge.
Such a magnetic brush developer roll must, of course, extend completely across that length of the photoconductor which contains a latent image. The entire length of this development nip must be continuously supplied with developer mix having sufficient toner, of a proper charge, to adequately develop whatever pattern latent image may exist on the photoconductor.
The present invention is directed to a magnetic brush developer whose developer mix is continuously recirculated by a supply and a return auger of unique construction. Specifically, these two augers have a multiflute auger construction whose pitch and speed insure adequate mix agitation for proper triboelectric charging, and insure an adequate supply of toner-rich mix along the length of the developer roll.
More specifically, the entire length of the magnetic brush roll is supplied with developer mix by a supply auger. This auger is longer than the brush roll and its discharge end is substantially flush with one end of the brush roll. The discharge end of the supply auger operates to deposit depleted carrier into a first turnaround compartment whereat new toner is added as needed. The intake end of the supply auger extends beyond the other end of the brush roll, into a second turnaround compartment.
A return auger of substantially the same length as the supply auger is axially offset therefrom, so that its exit end is substantially flush with said other end of the brush roll. The intake end of the return auger extends beyond said one end of the brush roll and operates to transport the depleted carrier and new toner from the first turnaround compartment to the exit end of the return auger, mixing and charging the same as it does so. The exit end of the return auger communicates with the second turnaround compartment whereat the now enriched and charged mix returns to the intake end of the supply auger for presentation to the brush roll.
The function of the supply auger is to insure a uniform supply of toner-rich developer mix along the entire length of the brush roll. As can be appreciated, a given carrier bead will be somewhat depleted of toner each time it is presented to the photoconductor. Thus, the supply auger is constructed and arranged to present a given carrier bead to the photoconductor a predetermined number of times as it travels the length of the supply auger. Exemplary, the supply auger contains four flutes of a pitch equal to one-half the length of the brush roll.
The primary function of the return auger is to insure proper mixing and triboelectric charging of the toner and carrier beads. Exemplary, the return auger includes two flutes of a pitch equal to the pitch of the supply auger.
Since the recirculating developer mix must have the same flow rate through both augers, their rotary speeds are related as are their diameters, the number of flutes, and the flute pitch. Preferably, the augers are made of material selected to have little or no affinity for either the charged toner or the charged carrier.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawing.